Male sex bias in Eiders Somateria mollissima during spring migration into the Gulf of Finland

Abstract

We recorded the sex ratio of adult Eiders migrating into the Gulf of Finland during spring (March–May) 2001 by visual observations at Hanko Bird Observatory, SW Finland. A total of 22 509 males and 17 234 females were sexed during migration, giving a highly significant departure from an even sex ratio (P < 0.001). The count thus indicated 56.6% males in the migrating population. Counts of local breeding birds in the vicinity of the bird observatory in April averaged a sex ratio of 55.9% ± 0.09% males in the flocks before median laying (range 54.1%–59.0%). Thus, both on a local and regional scale, the population of Eider in the Gulf of Finland is male dominated. The observed sex ratio is roughly similar to sex ratios observed in the wintering area in Danish waters, but deviates from previous, local, reports from the Finnish coast, suggesting an even sex ratio. The timing of the most pronounced male excess in the migrating flocks suggests that unpaired males move into the Gulf, rather than males that have terminated the bond with an already incubating female. We discuss the plausibility of alternative hypotheses for the observed male bias.